1. Field of the Invention
The invention relates generally to a gas turbine engine/generator, and more particularly toward a rotary ramjet turbo-generator.
2. Related Art
In a traditional Brayton-cycle gas turbine engine, a bladed compressor first compresses air, which then mixes and burns with a fuel like jet fuel, kerosene, natural gas or propane in a combustor. The heat that comes from the burning fuel expands the combustion product gases. As these hot product gases flow at high speed through a bladed turbine, a torque is produced on a turbine shaft. The torque is used to drive the compressor and possibly one or more external implements like a generator. These conventional Brayton-cycle engines require a bladed compressor, driven by a bladed turbine, to achieve the high compression ratios needed for acceptable operating efficiencies and power output. Most of the mass in such a conventional system results from the bladed compressor and turbine, which in turn limits the mass-specific power achievable. It has been a long-standing goal to develop more effective ways of obtaining substantially higher mass-specific power for Brayton-cycle engines.
Additionally, the need for a bladed compressor and turbine introduces severe limits on the performance of a conventional gas turbine engine. These limits stem from the high-temperature material limitations associated with the first-stage turbine blades. In particular a bladed turbine must be made from a material with good mechanical properties but relatively poor thermal properties which in turn imposes strict limits on the maximum turbine inlet temperature, and thus on the maximum combustor exit temperature. The maximum combustor exit temperature is commonly labeled T3 on a traditional temperature-entropy (T-s) diagram for the Brayton cycle. This practical limitation imposed by the material characteristics in the first-stage turbine blades lowers both the thermal efficiency and the power achievable from the conventional approach. It has thus been a further long-standing goal to develop effective ways to allow continuous operation of Brayton-cycle engines with substantially higher T3 values to provide both increased thermal efficiencies and further increased mass-specific power.
Furthermore, there has been interest in miniaturizing Brayton-cycle engines for various small-scale applications. However, miniaturization of conventional, large-scale turbine engines while maintaining comparably high operational performance has to date proven difficult. Most engineering issues encountered in large-scale gas turbines, such as the need to maintain acceptable compressor and turbine blade tip clearances, are even more difficult to address at small scales. The need for high-efficiency multi-stage bladed compressors and turbines also contributes significantly to the complexity and overall cost of producing small-scale turbine engines and generators based on miniaturization of conventional large-scale systems, and furthermore ultimately limits the service life of such engines. Current small-scale Brayton-cycle engines based on miniaturization of conventional large-scale gas turbine engines have been demonstrated to achieve substantially lower thermal efficiencies and mass-specific power than to their large-scale counterparts. It has thus been a further long-standing goal to develop effective ways to miniaturize Brayton-cycle engines to achieve higher performance, longer life, and lower cost than have to date been possible by direct miniaturization of traditional large-scale gas turbine engines.
In contrast to a traditional gas turbine engine, a traditional ramjet engine has no moving parts, and in particular has no bladed compressor or turbine. While also operating on a Brayton-cycle, the ramjet engine instead achieves compression of intake air by the ram pressure that develops from the relative difference in velocity between the engine and the intake air. A conventional ramjet engine consists of a specially shaped flow channel that is open at both ends, with the air used for combustion being rammed into the flow channel and compressed by the high forward motion of the engine relative to the surrounding air. The air entering this channel flows first through a diffuser section where, at initially supersonic velocities relative to the flow channel, compression takes place. The air speed is reduced to subsonic velocities before reaching the end of the diffuser section, whereupon it enters a combustor section in which it is mixed and burned with gaseous or atomized liquid fuel. The resulting hot combustion product gases continue flowing from the combustor into a nozzle section, where they expand through a supersonic nozzle to finally issue at supersonic velocity in a jet from the rear opening of the channel. The difference in the total momentum flow rate of the air and fuel entering the ramjet flow channel and the combustion product gases issuing from the rear opening of the flow channel produces a thrust force directed generally along the direction of the flow through the flow channel.
Thus, in the context of conventional ramjet engines, the use of an appropriately shaped supersonic diffuser and an appropriately shaped supersonic nozzle eliminates the need for a conventional bladed compressor and turbine, providing the benefits noted above. A further advantage of the ramjet engine arises out of the fact that its relatively simple flow channel shape can be more easily fabricated from high-temperature materials, such as, for example, ceramic materials that permit higher operating temperatures and thus greater efficiencies. However, conventional ramjet engines can only be used when the forward speed of the engine is sufficiently high relative to the surrounding air to produce suitable ram pressure which is needed for acceptable performance.
To overcome this practical limitation, rotary ramjet-based engines have been proposed as turbo generators for applications that include large-scale stationary power generation at up to about 9 megawatts using methane, waste gases, or other fuels. For example, numerous patents assigned to Ramgen Power Systems, Inc. of Bellevue, Wash. teach various ways in which to operate a large-scale rotary ramjet engine. Representative examples include Ramgen's U.S. Pat. Nos. 6,263,660 and 6,347,507, both in the name of Lawlor. Another exemplary source for large-scale rotary ramjet teachings may be found in US Patent Publication No. 2004/0020185 to Brouillette and Plante.
These prior art systems configure the compression-combustion-expansion channels that provide the ramjet flow path in a helical shape. These twisting flow channels are formed by an appropriately shaped radially outward surface of an inner supersonic rotating rotor that faces toward the radially inward surface of an outer stationary stator. Providing for only a small gap between these surfaces causes the flow channels formed from the radially outward surface of the rotor to be operably closed against the radially inward surface of the stator. This creates ramjet channels that remain operably open only at their upstream and downstream ends. By turning the rotor about its central axis at a sufficiently high rotation rate, the radially outward facing ramjet channels within the rotor move at sufficiently high supersonic speed relative to air, which is separately forced to flow axially at much lower speed toward the rotor and into the ramjet channels, to achieve sufficient ram pressure and accomplish ramjet performance. In principle, such prior art configurations for rotary ramjet engines eliminate the need for a bladed compressor and turbine in a stationary system. As such, they would appear to permit substantial improvements in the thermal efficiency, mass-specific power, service life, and cost of a Brayton-cycle engine as compared with its bladed cousin.
However, the prior art rotary ramjet configurations have numerous disadvantages that have to date prevented successful realization for lack of useful operating performance. Most notably, the rotor material within which the ramjet flow channels are formed is subjected to large tensile stresses similar to those experienced by a bladed turbine engine. These large stresses are the result of the radially outward orientation of the working surface of the rotor and the large centrifugal forces produced by the rotor motion. These tensile stresses arise because the rotor material is not constrained at its radially outermost surface from expanding radially outward under the large centrifugal forces that act on it. Just like in a bladed turbine engine, the resulting large tensile stresses in the rotating parts of a ramjet engine prevent high-temperature materials from being used to fabricate the rotor used in the ramjet flow channel because the ceramic material would likely fail in tension mode.
The desirable high-temperature materials spoken of here might include ceramics, ceramic-metallics, and similar materials that can withstand far higher temperatures than conventional turbine materials without substantially losing their material strength. A further characteristic of the desirable high-temperature materials is that they typically have far greater compressive strength than tensile strength, and in particular fail far more readily in tension than do the kinds of materials found in conventional bladed turbine engines. For this reason, conventional materials with high tensile strength are used in prior art to form the flow channels in a rotating ramjet system. The trade-off is unfortunate, because higher tensile strength materials cannot sustain high-temperatures, and therefore the maximum combustor exit temperatures (T3) which can be sustained in the prior art systems are substantially low relative to a stoichiometric ideal. Since the combustor exit temperature is a key factor in determining the power output and efficiency of a Brayton-cycle engine, the inability to use high-temperature materials (like ceramics) is a serious disadvantage of all prior art systems.
Accordingly, there is a need within the rotary ramjet engine field to enable the use of high-temperature materials, such as ceramics or ceramic-metallics and thereby allow operation at higher combustor exit temperatures (T3), which in turn will allow higher mass-specific power and higher thermal efficiency than are possible with conventional gas turbine engines or with prior art rotary ramjet engines.